1. Field of the Invention
The present invention relates to an SOI (silicon on insulator) substrate and a manufacturing method thereof.
Note that a semiconductor device in this specification refers to all types of devices which can function by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic devices are all included in the category of the semiconductor device.
2. Description of the Related Art
With development of VLSI technology, lower power consumption and higher speed over the scaling law which can be realized by bulk single-crystalline silicon have been demanded. In order to satisfy these requirements, an SOI structure is receiving attention. This technology allows an active region (channel formation region) of a field effect transistor (FET), which has been formed of bulk single-crystalline silicon, to be formed of a single-crystalline silicon thin film. It is known that a field effect transistor manufactured using an SOI structure has lower parasitic capacitance than a field effect transistor manufactured using a bulk single-crystalline silicon substrate, which is advantageous in increasing speed.
As an SOI substrate, a SIMOX substrate or a bonded substrate is known. A SIMOX substrate is formed in the following manner: oxygen ions are implanted into a single-crystalline silicon substrate and heat treatment is performed at 1300° C. or higher to form a buried oxide film, so that a single-crystalline silicon thin film is formed on the surface and thus an SOI structure is obtained. In the SIMOX substrate, oxygen ion implantation can be controlled precisely and thus a single-crystalline silicon film can be formed with an even thickness; however, oxygen ion implantation takes much time and there are problems in time and cost. Furthermore, since the SOI structure is formed using one base substrate, it has not been possible to use a different kind of substrate from the base substrate as a supporting substrate.
A bonded substrate is formed in the following manner: two single-crystalline silicon substrates are bonded to each other with an insulating layer interposed therebetween and one of the single-crystalline silicon substrates is thinned, so that a single-crystalline silicon thin film is formed to obtain an SOI structure. As a thinning method, a hydrogen ion implantation separation method is known. A hydrogen ion implantation separation method is a method in which hydrogen ions are implanted into one single-crystalline silicon substrate to form a microbubble layer at a predetermined depth from a surface of the silicon substrate, and a thin single-crystalline silicon layer can be bonded to the other single-crystalline silicon substrate with use of the microbubble layer as a separation surface (see Patent Document 1: Japanese Published Patent Application No. 2000-124092).
Recently, there has been an attempt to form a single-crystalline silicon layer over a substrate having an insulating surface such as a glass substrate. For example, one known example of an SOI substrate in which a single-crystalline silicon layer is formed over a glass substrate is disclosed by the present applicant (see Reference 2: Japanese Published Patent Application No. H11-163363).
In the case of utilizing the hydrogen ion implantation separation method, the volume of a microbubble layer formed in a silicon substrate is changed by heat treatment and separation occurs along the microbubble layer, so that an SOI structure can be obtained. However, in order to change the volume of the microbubble layer and separate the silicon substrate along the microbubble layer, heat treatment needs to be performed at approximately 600° C.; accordingly, there has been a problem in reducing throughput in the separation step.